Memory

Run the top command to check the memory usage of each node in openGauss and analyze whether a performance bottleneck occurs due to high memory usage.

Checking Memory Usage

You can query the memory usage of the server in the following ways:

Run the top command to check the memory usage. Then, press Shift+M to sort memory on each node by memory size.

top - 11:38:26 up 2 days, 17:59, 10 users,  load average: 0.01, 0.05, 0.15
Tasks: 685 total,   1 running, 684 sleeping,   0 stopped,   0 zombie
%Cpu(s):  0.2 us,  0.2 sy,  0.0 ni, 99.7 id,  0.0 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem : 19740646+total, 23503420 free, 15947100 used, 15795595+buff/cache
KiB Swap:  8242172 total,  8242172 free,        0 used. 13366219+avail Mem

  PID USER PR  NI    VIRT    RES    SHR S  %CPU %MEM     TIME+ COMMAND
29838 omm  20   0 1373104 456904 175248 S   3.6  0.2  98:53.16 gaussdb
27789 omm  20   0  150732   4136   3216 S   0.0  0.0   0:00.00 gsql
45659 omm  20   0  117164   4052   1860 S   0.0  0.0   0:00.24 bash
 8087 omm  20   0  117164   4000   1848 S   0.0  0.0   0:00.05 bash
27459 omm  20   0  117160   4000   1848 S   0.0  0.0   0:00.04 bash
33619 omm  20   0  117120   3852   1740 S   0.0  0.0   0:00.04 bash
27282 omm  20   0  117120   3840   1728 S   0.0  0.0   0:00.03 bash
 9923 omm  20   0  158064   2932   1612 R   0.3  0.0   0:00.04 top

In the command output, focus on the memory usage (%MEM) occupied by the processes and the free memory of the system.

The parameters in the command output are described as follows:

• total: size of the total physical memory
• used: size of the used physical memory
• free: size of the free memory
• buffers: size of the virtual memory used by a process
• %MEM: usage of the memory used by a process
• VIRT: size of the virtual memory occupied by a process. The occupied virtual memory can be calculated using VIRT = SWAP + RES.
• SWAP: size of the memory that has been transferred to the swap partition
• RES: size of the memory that has not been transferred to the swap partition
• SHR: size of the shared memory

Analyzing Performance Parameters

1. Run the following command as user root to check the cache usage:

   free
   

   The command output is similar to the following:

                total       used       free     shared    buffers     cached
   Mem:       8038844    6336184    1702660          0     375896    2880912
   -/+ buffers/cache:    3079376    4959468
   Swap:      4192924          0    4192924
   

2. If the cache usage is too high, run the following command to enable the automatic cache clearing function:

   sh openGauss-server/src/bin/scripts/run_drop_cache.sh
   

   The openGauss-server code download address is https://gitee.com/opengauss/openGauss-server.git

3. You need to check the execution plan if the memory usage occupied is too high. Focus on the following items:

   • Whether improper JOIN sequences exist. For example, if JOIN is performed for multiple tables, the intermediate result set of the two tables associated preferentially is large during the execution plan execution, resulting in that the final execution cost is high.